Telomere and telomerase are key factors that regulate cell replicative lifespan. Cross-sectional analyses of telomere length in blood leukocytes show an age-related shortening, but the actual in vivo change of telomere length and its relationship with telomerase, cell composition, and health conditions in humans is not fully understood. Here, we report a longitudinal analysis of telomere length, telomerase activity, lymphocyte composition and health conditions in peripheral blood mononuclear cells (PBMC), lymphocytes, and monocytes at the beginning and at a five-year follow-up of 200 participants (age range at entry from 21 to 91years) from the Baltimore Longitudinal Study of Aging (BLSA). We observed multiple directional changes in telomere length in vivo. In PBMC, including lymphocytes and monocytes, 34% of the subjects showed a decrease, 56% did not change, and 10% exhibited an increase of telomere length. Telomerase activity declined with age in resting and activated T cells, and resting but not activated B cells. Percentages of naive T cells decreased and CD28- T cells increased with age throughout the adult life span and were positively and negatively correlated to telomere length in T cells, respectively. Finally, a significant portion of the observed telomere attrition with age was explained by declined telomerase activity, decreased naive cells, and the presence of specific health conditions (cancer and adiposity). These findings show that changes of telomere length in vivo with age are multi-directional and that telomere length is affected by telomerase activity, age-related changes in blood lymphocyte subsets and changes in health conditions. Telomeres acts as a key regulator of the capacity of cell divisions and immune systems rely on cell proliferation for its function. While it is well documented that telomere shortening eventually leads to the secession of cell division in vitro, it is not unknown if telomere shortening plays a role in age-associated decline of immune function in human. Here we compared immune response against the influenza vaccine in healthy elderly donors who had relative long or short telomere lengths of peripheral blood mononuclear cells (PBMC). Our data show that donors with long telomeres had 1) higher percentage of good antibody response and 2) better expansion of influenza-specific CD8+ T cells than donors with short telomere after influenza vaccination. These results provide evidence for the first time that telomere length may play a role in age-associated decline of adaptive immune response in human.